Plasma Reactor Coupled with Catalyst...temperature. Nader et al. [25] treated a mixed waste gas of...

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Article

Abatement of Toluene by Reverse-Flow NonthermalPlasma Reactor Coupled with Catalyst

Wenjun Liang * , Huipin Sun, Xiujuan Shi and Yuxue Zhu

Key Laboratory of Beijing on Regional Air Pollution Control, Beijing University of Technology, Beijing 100124,China; [email protected] (H.S.); [email protected] (X.S.); [email protected] (Y.Z.)* Correspondence: [email protected]; Tel.: +86-10-6739-2080

Received: 2 April 2020; Accepted: 3 May 2020; Published: 7 May 2020��

Abstract: In order to make full use of the heat in nonthermal plasma systems and decrease thegeneration of by-products, a reverse-flow nonthermal plasma reactor coupled with catalyst was usedfor the abatement of toluene. In this study, the toluene degradation performance of different reactorswas compared under the same conditions. The mechanism of toluene abatement by nonthermalplasma coupled with catalyst was explored, combined with the generation of ozone (O3), NO2, andorganic by-products during the reaction process. It was found that a long reverse cycle time of thereactor and a short residence time of toluene decreased the internal reactor temperature, which wasnot beneficial for the degradation of toluene. Compared with the dielectric barrier discharge (DBD)reactor, toluene degradation efficiency in the double dielectric barrier discharge (DDBD) reactor wasimproved at the same discharge energy level, but the concentrations of NO2 and O3 in the effluentwere relatively high; this was improved after the introduction of a catalyst. In the reverse-flownonthermal plasma reactor coupled with catalyst, the CO2 selectivity was the highest, while theselectivity and amount of NO2 was the lowest and aromatics, acids, and ketones were the maingaseous organic by-products in the effluent. The reverse-flow DBD-catalyst reactor was successful indecreasing organic by-products, while the types of organic by-products in the DDBD reactor weremuch more than those in the DBD reactor.

Keywords: DBD; DDBD; catalyst; toluene; by-products; nonthermal plasma; reverse-flow

1. Introduction

Volatile organic compounds (VOCs) are atmospheric pollutants that are harmful to human healthand the environment and have attracted increasing attention. Toluene is a typical VOC [1,2] that hasharmful effects on the human body, especially the nervous system.

Dielectric barrier discharge (DBD) is a common nonthermal plasma generation method [3] thatincludes two main discharge forms: dielectric barrier discharge (DBD) and double dielectric barrierdischarge (DDBD) [4,5]. According to the placement of catalysts, DBD can be divided into in-plasmacatalysis (IPC) and post-plasma catalysis (PPC). According to the discharge setup, DBD can be dividedinto volumetric DBD, surface DBD [6], and hybrid setups. The most commonly used DBD reactorsetup combined with a catalyst is volumetric DBD with IPC [7]. In experimental research, comparedwith plate-to-plate or parallel-plate DBD devices, annular or packed-bed DBD devices are widely usedin plasma catalysis research, due to their convenient set-up.

During the degradation of VOCs, conventional DBD reactors have some shortcomings, suchas low energy yield, harmful by-products, and poor selectivity [8]. In recent years, there have alsobeen many literature reports on the degradation of DDBD [9,10]. Some studies found that DDBDcan effectively improve the removal rate of pollutants [11]. Li et al. [12] found that in DDBD reactorsthe mineralization rate of γ-Al2O3 and ZSM-5 mixed packing was high, and rod electrodes had

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Catalysts 2020, 10, 511 2 of 17

superior toluene decomposition performance compared to coil electrodes. Zhang et al. [13] foundthat there was less organic by-product generation in a DDBD reactor. Mustafa et al. [14] found thata DDBD reactor with a 3 mm discharge gap resulted in better degradation of pollutants than whenthe discharge gap was 6 mm, and the degradation of pollutants was improved after the addition ofcatalyst. Muhammad et al. [15] found that in a DDBD plasma reactor the generation of ozone wasclosely related to the discharge current, and the concentration of ozone increased exponentially in theoxygen-containing atmosphere. Therefore, for some pollutants, the degradation effect of DDBD andDBD reactors depended on the specific experimental conditions.

However, nonthermal plasma technologies share many problems, including generation ofby-products and high-energy consumption [16]. Over past decades, in order to offset these weaknesses,plasma-catalytic technology has received considerable attention as a promising method for the completeremoval of VOCs [17–19]. This method combines benefits from the rapid response time of the plasmatechnique and the high selectivity of catalysis [20,21]. Adding catalysts to DBD reactors is not onlybeneficial to the degradation of pollutants, but also in decreasing the generation of by-products [22,23].Siddharth et al. [24] found that under a given electric potential, the cooperative catalytic systemcould polarize more effectively, enhance the electric field intensity and increase the average electrontemperature. Nader et al. [25] treated a mixed waste gas of benzene, toluene, and xylene with asynthesized catalyst and nonthermal plasma, effectively improving the degradation efficiency ofpollutants. Zhu et al. [26] filled a nanometer-scale catalyst into the discharge area, greatly decreasingthe output of by-products O3 and NO2. At present, the combination of catalytic oxidation technologyand nonthermal plasma technology is one of the core technologies for the treatment of VOCs [27,28].

In the field of catalytic oxidation, flow reversing technology has been widely used because itcan effectively utilize system heat to achieve heat accumulation and increase energy yield [29–31].Krzysztof et al. [32] optimized the thermal flow thermal reactor by increasing the gas flowrate throughmore internal channels and extending the length of the inclined part of the roof to reduce the occurrenceof turbulence, which improved the flow uniformity, and the experimental results were consistent withthe simulation results. In a pilot scale reactor, Liang et al. [33] applied flow direction conversion tokeep the reactor at a high temperature; this decreased heat loss via exhaust gas and kept the reactoroperating at a low concentration of raw gas.

Clearly, flow reversing technology can promote the effective use of heat in the system, which is agood way to save energy and improve energy yield. Therefore, it was decided to try combining flowreversing technology with a DBD reactor. In preliminary investigations it was found that there werefew studies that combined the use of the three technologies. On the basis of previous experiments,it appeared that flow reversing technology could be introduced to enable the use of the heat generatedby the plasma discharge instead of external heating, and that it could further solve the problem oflow degradation efficiency and energy yield in the nonthermal plasma treatment of VOCs. Lianget al. [34,35] found that in a flow reversing plasma reactor, the system temperature increased andchanged periodically with the changes in the reversing period. Compared with conventional DBDreactors, the optimized system conditions were conducive to increasing the system temperature andthe degradation of pollutants. The higher discharge energy level, the higher the toluene degradationeffect; however, the higher the maximum ozone production as well.

In this study, in order to make full use of the heat in a nonthermal plasma system and decreasethe generation of by-products, a reverse-flow nonthermal plasma reactor coupled with catalysts wasused for the abatement of toluene. Through studying the influence of flow reversal parameters onenergy consumption, toluene degradation, and by-products of the reaction system, optimized flowdirection transformation parameters were obtained. The degradation path of toluene under two kindsof system conditions was also studied.

Catalysts 2020, 10, 511 3 of 17

2. Results and Discussion

2.1. Catalyst Characterization

In the experiment, 7.5 wt % Mn/cordierite catalyst was used to fill the discharge zone to participatein toluene degradation. The catalysts were characterized to study the chemistry and structure.

Textural parameters including the specific surface area (SBET), pore diameter (Pd), and pore volume(Pv) were determined. As shown in Table 1, the SBET of original cordierite was very small at 1.5 m2·g−1,and there was almost no surface pore structure. After pre-treatment, the SBET increased to 55.1 m2·g−1,the Pv increased to 0.049 cm3·g−1 and the Pd decreased, indicating that a large number of pore structuresappeared on the carrier surface, which was conducive to the loading of active components. The SBETdecreased to 24.2 m2·g−1 and the Pd increased by 4.2 nm after the active component was supported,indicating that the active component occupied a larger pore in the carrier and was better loaded on thesurface of the carrier.

Table 1. The specific surface area and pore structure of catalysts.

Catalysts SBET (m2·g−1) Pd (nm) Pv (cm3·g−1)Original cordierite 1.5 17.7 0.0003

Cordierite (after pre-treatment) 55.1 4.1 0.0497.5 wt % Mn/cordierite catalyst 24.2 8.3 0.047

In order to observe the loading of the active components on the carrier surface, three sampleswere further analyzed by scanning electron microscope (SEM). In the Figure 1, the surface of theoriginal cordierite was relatively flat. After pre-treatment, a silica coating was observed on the carriersurface. After the loading of active components, the particles were uniformly covered with silica andthe number of reactive sites was increased, which was conducive to toluene degradation.

Catalysts 2020, 10, x FOR PEER REVIEW 3 of 17

Catalysts2020, 10, x; doi: FOR PEER REVIEW www.mdpi.com/journal/catalysts

In the experiment, 7.5 wt% Mn/cordierite catalyst was used to fill the discharge zone to

participate in toluene degradation. The catalysts were characterized to study the chemistry and

structure.

Textural parameters including the specific surface area (SBET), pore diameter (Pd), and pore

volume (Pv) were determined. As shown in Table 1, the SBET of original cordierite was very small at

1.5 m2·g1, and there was almost no surface pore structure. After pre-treatment, the SBET increased to

55.1 m2·g1, the Pv increased to 0.049 cm3·g1 and the Pd decreased, indicating that a large number of

pore structures appeared on the carrier surface, which was conducive to the loading of active

components. The SBET decreased to 24.2 m2·g1 and the Pd increased by 4.2 nm after the active

component was supported, indicating that the active component occupied a larger pore in the

carrier and was better loaded on the surface of the carrier.

Table 1. The specific surface area and pore structure of catalysts.

Catalysts SBET (m2·g1) Pd (nm) Pv (cm3·g1)

Original cordierite 1.5 17.7 0.0003

Cordierite (after pre-treatment) 55.1 4.1 0.049

7.5 wt% Mn/cordierite catalyst 24.2 8.3 0.047

In order to observe the loading of the active components on the carrier surface, three samples

were further analyzed by scanning electron microscope (SEM). In the Figure 1, the surface of the

original cordierite was relatively flat. After pre-treatment, a silica coating was observed on the

carrier surface. After the loading of active components, the particles were uniformly covered with

silica and the number of reactive sites was increased, which was conducive to toluene degradation.

(a) (b) (c)

Figure 1. Scanning electron microscope (SEM) images of (a) original cordierite, (b)cordierite (after

pre-treatment), and (c) 7.5 wt% Mn/cordierite catalyst.

To determine the actual content of active components on the catalyst, three samples were

tested by inductively coupled plasma optical emission spectroscopy (ICP-OES). Two parallel

samples were tested, and the average value was taken as the test result (Table 2). It can be seen from

Table 2 that the actual load of Mn was lower than that calculated theoretically, indicating that there

was a certain amount of loss of active components during catalyst preparation.

Table 2. Elemental analysis results for the catalysts.

Catalysts Test Times Element Calculated Value (%) Actual Value (%)

7.5 wt% Mn/cordierite catalyst

1 Mn 7.5 7.2

2 Mn 7.5 6.8

3 Mn 7.5 7.1

2.2. Degradation of Toluene in DBD and DDBD Reactor

Figure 1. Scanning electron microscope (SEM) images of (a) original cordierite, (b) cordierite (afterpre-treatment), and (c) 7.5 wt % Mn/cordierite catalyst.

To determine the actual content of active components on the catalyst, three samples were testedby inductively coupled plasma optical emission spectroscopy (ICP-OES). Two parallel samples weretested, and the average value was taken as the test result (Table 2). It can be seen from Table 2 thatthe actual load of Mn was lower than that calculated theoretically, indicating that there was a certainamount of loss of active components during catalyst preparation.

Table 2. Elemental analysis results for the catalysts.

Catalysts Test Times Element Calculated Value (%) Actual Value (%)

7.5 wt %Mn/cordierite

catalyst

1 Mn 7.5 7.22 Mn 7.5 6.83 Mn 7.5 7.1

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2.2. Degradation of Toluene in DBD and DDBD Reactor

At a set energy level (250 J·L−1), the degradation of toluene was carried out and the performanceof DBD and DDBD reactors was compared in terms of discharge and degradation efficiency. The flowdirection conversion period was 8 min per cycle and the gas flow rate was 6 L·min−1. The fivekinds of reactors tested were the: (I) DBD reactor; (II) DBD-catalyst reactor; (III) DDBD reactor; (IV)DDBD-catalyst reactor; and (V) reverse-flow DBD-catalyst reactor. Points P and Q are the temperaturemeasurement points of the heat storage section of the reactor, and point O is at the discharge area inFigure 2a.



At a set energy level (250 J·L1), the degradation of toluene was carried out and the

performance of DBD and DDBD reactors was compared in terms of discharge and degradation

efficiency. The flow direction conversion period was 8 min per cycle and the gas flow rate was 6

L·min1. The five kinds of reactors tested were the: (I) DBD reactor; (II) DBD-catalyst reactor; (III)

DDBD reactor; (IV) DDBD-catalyst reactor; and (V) reverse-flow DBD-catalyst reactor. Points P and

Q are the temperature measurement points of the heat storage section of the reactor, and point O is

at the discharge area in Figure 2a.

In the experiments, DDBD reactors were found to decrease the discharge energy density under

the same discharge voltage compared with DBD reactors. This was due to the increase of the

dielectric layer thickness in the tube and the decrease of the discharge space in the system, so the

comparison was carried out under the same discharge energy density.

0

20

40

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120

(Ⅰ) (Ⅱ) (Ⅲ) (Ⅳ) (Ⅴ)

P

O

Q

T (℃

)

0

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80

(Ⅰ) (Ⅱ) (Ⅲ) (Ⅳ) (Ⅴ)

η (

%)

η EY

0

1

2

3

4

5

6

EY

(g /

(kW·

h)

)

(a) (b)

Figure 2. Toluene degradation in different reactors. (a) Changes in T; (b) changes in η and energy

yield (EY).

From Figure 2a, it can be seen that the temperature in the conventional DBD reactor (I) was low,

while the temperature in the DDBD reactor (III) was relatively high. The difference between the

three temperature measuring points in the latter reactor was small. The temperature of the DDBD

reactor with an empty tube was relatively high, up to 85.3 °C. In the DDBD reactor, the airflow

entered the outer tube after entering the buffer stable flow field. The change of air-flow direction in

the inner and outer tubes led to a relatively consistent increase in the temperature of the system.

Therefore, compared with the DBD reactor, the temperature of the DDBD reaction zone was higher.

Due to the existence of multiple dielectric layers in the DDBD reactors, the discharge of the

inner tube was more intense, forming a strong electric field area as the outer tube formed a weak

electric field area. With the same energy input, pollutants passed through the two discharge areas of

the strong electric field in the inner tube and the weak electric field in the outer tube, so that the

toluene conversion was improved by 31.6% (Figure 2b). However, compared with conventional

DBD reactor, the distribution of strong and weak electric fields in DDBD reactors resulted in the

formation of more organic by-products. The gas velocity was higher in the inner pipe, and the

reaction time of pollutants passing through the strong electric field was shorter. As previously

reported, short residence time decreases the removal efficiency of VOCs [36,37]. Some pollutant

molecules were initially destroyed by collision with high-energy particles. However, due to the

insufficient reaction time, some pollutant molecules left the strong discharge area before being

completely degraded, leading to the formation of a large number of organic by-products that

attached to the inner tube wall.

In the DDBD-catalyst reactor (IV), the surface of the catalyst was readily covered by organic

by-products. This made it difficult for the catalyst to play its role as there was a synergistic effect

between the catalyst and plasma only in the early stage of the reaction, which gradually weakened

with the extension of discharge time. Therefore, the DDBD-catalyst reactor could not effectively

improve the degradation efficiency of pollutants or the energy yield of the system.

Figure 2. Toluene degradation in different reactors. (a) Changes in T; (b) changes in η and energyyield (EY).

In the experiments, DDBD reactors were found to decrease the discharge energy density underthe same discharge voltage compared with DBD reactors. This was due to the increase of the dielectriclayer thickness in the tube and the decrease of the discharge space in the system, so the comparisonwas carried out under the same discharge energy density.

From Figure 2a, it can be seen that the temperature in the conventional DBD reactor (I) was low,while the temperature in the DDBD reactor (III) was relatively high. The difference between the threetemperature measuring points in the latter reactor was small. The temperature of the DDBD reactorwith an empty tube was relatively high, up to 85.3 ◦C. In the DDBD reactor, the airflow entered theouter tube after entering the buffer stable flow field. The change of air-flow direction in the innerand outer tubes led to a relatively consistent increase in the temperature of the system. Therefore,compared with the DBD reactor, the temperature of the DDBD reaction zone was higher.

Due to the existence of multiple dielectric layers in the DDBD reactors, the discharge of the innertube was more intense, forming a strong electric field area as the outer tube formed a weak electricfield area. With the same energy input, pollutants passed through the two discharge areas of thestrong electric field in the inner tube and the weak electric field in the outer tube, so that the tolueneconversion was improved by 31.6% (Figure 2b). However, compared with conventional DBD reactor,the distribution of strong and weak electric fields in DDBD reactors resulted in the formation of moreorganic by-products. The gas velocity was higher in the inner pipe, and the reaction time of pollutantspassing through the strong electric field was shorter. As previously reported, short residence timedecreases the removal efficiency of VOCs [36,37]. Some pollutant molecules were initially destroyedby collision with high-energy particles. However, due to the insufficient reaction time, some pollutantmolecules left the strong discharge area before being completely degraded, leading to the formation ofa large number of organic by-products that attached to the inner tube wall.

In the DDBD-catalyst reactor (IV), the surface of the catalyst was readily covered by organicby-products. This made it difficult for the catalyst to play its role as there was a synergistic effectbetween the catalyst and plasma only in the early stage of the reaction, which gradually weakened with

Catalysts 2020, 10, 511 5 of 17

the extension of discharge time. Therefore, the DDBD-catalyst reactor could not effectively improvethe degradation efficiency of pollutants or the energy yield of the system.

For the DBD-catalyst reactor (II), Figure 2b shows that with increasing system temperature, thedegradation rate of toluene increased by 12.7% and the energy field increased by 1.6 g·kW−1·h−1.This indicated that the introduction of a catalyst was beneficial to the purification of pollutants,showing a certain synergistic effect. On the catalyst surface, the adsorbed pollutants were degradedby high-energy electrons and active radicals. Meanwhile, ozone and other long-lived substanceswith oxidative ability were adsorbed on the catalyst surface, which also promoted the degradation oftoluene. However, the degradation effect of pollutants still needed to be improved in the DDBD reactor.

Considering the serious secondary pollution caused by the generation of organic by-productsin the DDBD reactor, the flow reversal technology was connected in series with the DBD reactor toenable change of flow direction and optimization of the reaction system. Most of the heat generatedby the discharge of a single DBD reactor was carried out of the reactor by air flow. However, in thereverse-flow DBD-catalyst reactor (V), the heat generated by plasma discharge or released by catalyticoxidation further increased the system temperature. As shown in Figure 2a,b, the heat generatedby the flow direction conversion technology oscillated back and forth in the system, achieving amaximum system temperature of 105.8 ◦C. This effectively activated the catalyst and improved thecatalytic oxidation efficiency and energy yield (EY) of the system, achieving improvement by 38.1%and 2.93 g·kW−1·h−1.

In the process of catalytic degradation of toluene in the nonthermal plasma systems, the mixtureof toluene was obtained by purging with air, so the formation of ozone was inevitable. Figure 3 showsthat the concentration of ozone generated in the DDBD reactor was significantly higher than that inthe DBD reactor. The concentration of ozone was lowest in the reverse-flow DBD-catalyst reactor,at 162.9 mg·m−3, as some of the ozone was decomposed in the later stage of the reaction. In theDDBD reactor, the concentration of O3 increased gradually with the extension of the discharge time,finally reaching 448.4 mg·m−3. At this time, part of the ozone participated in the reaction or thermaldecomposition, resulting in a slightly decreased concentration. However, in the DDBD reactor, due tothe weak field strength in the discharge area of the outer tube and the low-temperature increase of thesystem, the ozone concentration was much higher than that in the DBD reactor. When catalyst wasadded into the reactor, the formation of O3 was inhibited [38] and the concentration of O3 decreased.The catalyst could absorb ozone on its active sites to achieve catalytic degradation of ozone, effectivelydecreasing the concentration of O3 and ultimately decreasing secondary pollution [39].



For the DBD-catalyst reactor (II), Figure 2b shows that with increasing system temperature, the

degradation rate of toluene increased by 12.7% and the energy field increased by 1.6 g·kW1·h1. This

indicated that the introduction of a catalyst was beneficial to the purification of pollutants, showing

a certain synergistic effect. On the catalyst surface, the adsorbed pollutants were degraded by

high-energy electrons and active radicals. Meanwhile, ozone and other long-lived substances with

oxidative ability were adsorbed on the catalyst surface, which also promoted the degradation of

toluene. However, the degradation effect of pollutants still needed to be improved in the DDBD

reactor.

Considering the serious secondary pollution caused by the generation of organic by-products in

the DDBD reactor, the flow reversal technology was connected in series with the DBD reactor to

enable change of flow direction and optimization of the reaction system. Most of the heat generated

by the discharge of a single DBD reactor was carried out of the reactor by air flow. However, in the

reverse-flow DBD-catalyst reactor (V), the heat generated by plasma discharge or released by

catalytic oxidation further increased the system temperature. As shown in Figure 2a,b, the heat

generated by the flow direction conversion technology oscillated back and forth in the system,

achieving a maximum system temperature of 105.8 °C. This effectively activated the catalyst and

improved the catalytic oxidation efficiency and energy yield (EY) of the system, achieving

improvement by 38.1% and 2.93 g·kW1·h1.

In the process of catalytic degradation of toluene in the nonthermal plasma systems, the

mixture of toluene was obtained by purging with air, so the formation of ozone was inevitable.

Figure 3 shows that the concentration of ozone generated in the DDBD reactor was significantly

higher than that in the DBD reactor. The concentration of ozone was lowest in the reverse-flow

DBD-catalyst reactor, at 162.9 mg·m3, as some of the ozone was decomposed in the later stage of

the reaction. In the DDBD reactor, the concentration of O3 increased gradually with the extension of

the discharge time, finally reaching 448.4 mg·m3. At this time, part of the ozone participated in the

reaction or thermal decomposition, resulting in a slightly decreased concentration. However, in the

DDBD reactor, due to the weak field strength in the discharge area of the outer tube and the

low-temperature increase of the system, the ozone concentration was much higher than that in the

DBD reactor. When catalyst was added into the reactor, the formation of O3 was inhibited [38] and

the concentration of O3 decreased. The catalyst could absorb ozone on its active sites to achieve

catalytic degradation of ozone, effectively decreasing the concentration of O3 and ultimately

decreasing secondary pollution [39].

0

100

200

300

400

500

(Ⅰ) (Ⅱ) (Ⅲ) (Ⅳ) (Ⅴ)

O3

(m

g

m-3

)

Figure 3. Concentration of O3 formed in different reactors.

2.3. Toluene Degradation in Reverse-Flow DBD-Catalyst Reactor

Figure 3. Concentration of O3 formed in different reactors.

Catalysts 2020, 10, 511 6 of 17

2.3. Toluene Degradation in Reverse-Flow DBD-Catalyst Reactor

In the reverse-flow DBD-catalyst reactor, toluene degradation experiments were carried outunder different reaction system conditions. Furthermore, in order to study the role of flow reversingtechnology on the reverse-flow DBD-catalyst reactor, toluene degradation was investigated underdifferent flow reversing parameters.

2.3.1. Effect of Flow Reversing Cycle Time

The flow reversing cycle is one of the most important factors affecting reactor performance. Whenthe flow direction switches, the heat oscillates in the reactor [40] where it can heat the intake air andensure the preheating of the catalyst bed [41]. The effect of the flow reversing cycle time on toluenedegradation was investigated with a discharge voltage of 16 kV, frequency of 100 Hz, and residencetime of 0.33 s. By selecting the appropriate commutation period, effective heat accumulation can beachieved [42].

Compared with the conventional DBD reactor, it was found that the central temperature ofposition O increased markedly after reversing the flow direction, resulting in more heat being stored inthe reaction zone. From Figure 4 it can be seen that when the flow reversing cycle time was 8 min, themaximum temperature of the discharge area could reach 105.8 ◦C, which was about 50 ◦C above thetemperature at both ends, and the discharge area temperature was 39.6 ◦C higher than when the flowreversing cycle time was 16 min. As the flow reversing cycle time was further increased, the flow wasmaintained in a single direction for a long time. This meant that the heat was easily carried out by theair flow, which was not conducive to the accumulation of heat and resulted in the gradual decline ofoverall temperature in the system and a large amount of wasted energy. This, in turn, weakened therole of the catalyst in the synergistic degradation of toluene.



In the reverse-flow DBD-catalyst reactor, toluene degradation experiments were carried out

under different reaction system conditions. Furthermore, in order to study the role of flow reversing

technology on the reverse-flow DBD-catalyst reactor, toluene degradation was investigated under

different flow reversing parameters.

2.3.1. Effect of Flow Reversing Cycle Time

The flow reversing cycle is one of the most important factors affecting reactor performance.

When the flow direction switches, the heat oscillates in the reactor [40] where it can heat the intake

air and ensure the preheating of the catalyst bed [41]. The effect of the flow reversing cycle time on

toluene degradation was investigated with a discharge voltage of 16 kV, frequency of 100 Hz, and

residence time of 0.33 s. By selecting the appropriate commutation period, effective heat

accumulation can be achieved [42].

Compared with the conventional DBD reactor, it was found that the central temperature of

position O increased markedly after reversing the flow direction, resulting in more heat being stored

in the reaction zone. From Figure 4 it can be seen that when the flow reversing cycle time was 8 min,

the maximum temperature of the discharge area could reach 105.8 °C, which was about 50 °C above

the temperature at both ends, and the discharge area temperature was 39.6 °C higher than when the

flow reversing cycle time was 16 min. As the flow reversing cycle time was further increased, the

flow was maintained in a single direction for a long time. This meant that the heat was easily carried

out by the air flow, which was not conducive to the accumulation of heat and resulted in the gradual

decline of overall temperature in the system and a large amount of wasted energy. This, in turn,

weakened the role of the catalyst in the synergistic degradation of toluene.

8 10 12 14 16

0

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120

P O Q

T (℃

)

Flow reversing cycle time (min)

8 10 12 14 16

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η EY

η (

%)


0

1

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4

5

6

EY

(g

k

W-1

h-1

)

(a) (b)

Figure 4. Effect of flow reversing cycle on reverse-flow dielectric barrier discharge (DBD)-catalyst

reactor capabilities: (a) changes in T; (b) changes in η and EY.

The degradation rate of toluene and the energy yield of the system were also negatively related

to the reversing cycle time. With a 16 min cycle time, the degradation rate of toluene decreased by

31%, while the energy yield of the system decreased by 1.84 g·kW1·h1. Therefore, the extension of

flow reversing cycle time resulted in increased release of heat generated in the plasma reactor and

the rapid annihilation of high-energy electrons, which led to decreased toluene degradation rate and

energy yield. Therefore, the optimal flow reversing cycle time was 8 min, which produced the

system with the highest heat utilization rate and afforded the best toluene degradation effect.

The effect of the reversing cycle on ozone generation is shown in Figure 5. As the reaction

system was equipped with the flow reversing technology, part of the heat generated by the plasma

discharge was stored in the heat storage section [43]. Since the internal temperature of the reaction

system could reach up to 100 °C, the ozone was decomposed by heat while participating in toluene

Figure 4. Effect of flow reversing cycle on reverse-flow dielectric barrier discharge (DBD)-catalystreactor capabilities: (a) changes in T; (b) changes in η and EY.

The degradation rate of toluene and the energy yield of the system were also negatively related tothe reversing cycle time. With a 16 min cycle time, the degradation rate of toluene decreased by 31%,while the energy yield of the system decreased by 1.84 g·kW−1·h−1. Therefore, the extension of flowreversing cycle time resulted in increased release of heat generated in the plasma reactor and the rapidannihilation of high-energy electrons, which led to decreased toluene degradation rate and energyyield. Therefore, the optimal flow reversing cycle time was 8 min, which produced the system with thehighest heat utilization rate and afforded the best toluene degradation effect.

The effect of the reversing cycle on ozone generation is shown in Figure 5. As the reaction systemwas equipped with the flow reversing technology, part of the heat generated by the plasma dischargewas stored in the heat storage section [43]. Since the internal temperature of the reaction system could

Catalysts 2020, 10, 511 7 of 17

reach up to 100 ◦C, the ozone was decomposed by heat while participating in toluene degradation.The heat accumulation of the system was weakened with the extension of the reversing cycle time,resulting in increased ozone concentration.



degradation. The heat accumulation of the system was weakened with the extension of the

reversing cycle time, resulting in increased ozone concentration.

8 10 12 14 16

0

70

140

210

280

O3

(mg

m

-3)


Figure 5. Effect of flow reversing cycle time on O3 concentration.

2.3.2. Effect of Residence Time

Figure 6 shows the effects of air flow rate, and consequently, the residence time of toluene

molecules, with a discharge voltage of 16 kV, frequency of 100 Hz, and flow reversing cycle time of 8

min. Due to the introduction of flow direction reversing technology, the continuous change in air

flow direction resulted in part of the heat released by the discharge of the plasma reactor

accumulating in the heat storage section. As the air flow increased, the residence time of toluene

molecules in the reactor was shortened. The initial internal temperature of the system showed an

overall upward trend. When the gas volume was 8 L·min1, the temperature at point O was 48.2 °C,

and the highest temperature was 105.8 °C. When the residence time was longer than 0.35 s, the

toluene removal efficiency dropped sharply. The decrease in residence time increased the gas

velocity. The high-speed gas drove the heat in the reactor to change the heat concentration point

with the flow direction. When the residence time was longer than 0.33 s, the increase in gas volume

resulted in an increase in the number of toluene molecules, improving the utilization ratio of active

radicals and high-energy electrons. The system reacted violently and emitted more heat. When the

residence time was shorter than 0.33 s, this resulted in increased gas velocity, which led to the

aggregation point change and the temperature dropped sharply; when the residence time was too

short, the collector moved out of the regenerator and caused a sharp decrease in system temperature

and the disappearance of flow direction reversal. The effect was similar to that of the ordinary DBD

reactor.

4 5 6 7 8

0

30

60

90

120

T (℃

)

P O Q Residence Time

Gas volume (L/min)

0.0

0.1

0.2

0.3

0.4

0.5

0.6

Resi

den

ce t

ime (

s)

0.25 0.30 0.35 0.40 0.50

0

10

20

30

40

50

60

70

80

η EY

η (

%)

Residence time (s)

0

1

2

3

4

5

EY

(g

k

W-1

h-1

)

(a) (b)



Figure 6 shows the effects of air flow rate, and consequently, the residence time of toluenemolecules, with a discharge voltage of 16 kV, frequency of 100 Hz, and flow reversing cycle time of8 min. Due to the introduction of flow direction reversing technology, the continuous change in air flowdirection resulted in part of the heat released by the discharge of the plasma reactor accumulating in theheat storage section. As the air flow increased, the residence time of toluene molecules in the reactorwas shortened. The initial internal temperature of the system showed an overall upward trend. Whenthe gas volume was 8 L·min−1, the temperature at point O was 48.2 ◦C, and the highest temperaturewas 105.8 ◦C. When the residence time was longer than 0.35 s, the toluene removal efficiency droppedsharply. The decrease in residence time increased the gas velocity. The high-speed gas drove the heatin the reactor to change the heat concentration point with the flow direction. When the residence timewas longer than 0.33 s, the increase in gas volume resulted in an increase in the number of toluenemolecules, improving the utilization ratio of active radicals and high-energy electrons. The systemreacted violently and emitted more heat. When the residence time was shorter than 0.33 s, this resultedin increased gas velocity, which led to the aggregation point change and the temperature droppedsharply; when the residence time was too short, the collector moved out of the regenerator and causeda sharp decrease in system temperature and the disappearance of flow direction reversal. The effectwas similar to that of the ordinary DBD reactor.

As shown in Figure 6b, the degradation rate of toluene, the energy yield, and the temperature ofthe system firstly increased and then decreased with increased residence time. When the residencetime was shortened, the reaction time of toluene molecules was shortened, and the degradation rateof toluene decreased by 35%. The EY decreased by 0.15 g·kW−1·h−1, which was lower than in theDDBD reactor.

Catalysts 2020, 10, 511 8 of 17



degradation. The heat accumulation of the system was weakened with the extension of the

reversing cycle time, resulting in increased ozone concentration.

8 10 12 14 16

0

70

140

210

280

O3

(mg

m

-3)




Figure 6 shows the effects of air flow rate, and consequently, the residence time of toluene

molecules, with a discharge voltage of 16 kV, frequency of 100 Hz, and flow reversing cycle time of 8

min. Due to the introduction of flow direction reversing technology, the continuous change in air

flow direction resulted in part of the heat released by the discharge of the plasma reactor

accumulating in the heat storage section. As the air flow increased, the residence time of toluene

molecules in the reactor was shortened. The initial internal temperature of the system showed an

overall upward trend. When the gas volume was 8 L·min1, the temperature at point O was 48.2 °C,

and the highest temperature was 105.8 °C. When the residence time was longer than 0.35 s, the

toluene removal efficiency dropped sharply. The decrease in residence time increased the gas

velocity. The high-speed gas drove the heat in the reactor to change the heat concentration point

with the flow direction. When the residence time was longer than 0.33 s, the increase in gas volume

resulted in an increase in the number of toluene molecules, improving the utilization ratio of active

radicals and high-energy electrons. The system reacted violently and emitted more heat. When the

residence time was shorter than 0.33 s, this resulted in increased gas velocity, which led to the

aggregation point change and the temperature dropped sharply; when the residence time was too

short, the collector moved out of the regenerator and caused a sharp decrease in system temperature

and the disappearance of flow direction reversal. The effect was similar to that of the ordinary DBD

reactor.

4 5 6 7 8

0

30

60

90

120T

(℃

)

P O Q Residence Time

Gas volume (L/min)

0.0

0.1

0.2

0.3

0.4

0.5

0.6

Resi

den

ce t

ime (

s)

0.25 0.30 0.35 0.40 0.50

0

10

20

30

40

50

60

70

80

η EY

η (

%)

Residence time (s)

0

1

2

3

4

5

EY

(g

k

W-1

h-1

)

(a) (b)

Figure 6. Effect of the residence time on DBD reactor: (a) changes in T; (b) changes in η and EY.

As shown in Figure 7, the maximum concentration of ozone increased gradually as the residencetime increased. As the maximum temperature in the reaction system could reach above 100 ◦C,the ozone participated in toluene degradation and was decomposed by the heat [44]. As the residencetime increased, there were more reactions between pollutant molecules and active radicals in thereactor. At the same time, more active oxygen atoms reacted with each other to form ozone, but theincrease of ozone generation was slow.



Figure 6. Effect of the residence time on DBD reactor: (a) changes in T; (b) changes in η and EY.

As shown in Figure 6b, the degradation rate of toluene, the energy yield, and the temperature of

the system firstly increased and then decreased with increased residence time. When the residence

time was shortened, the reaction time of toluene molecules was shortened, and the degradation rate

of toluene decreased by 35%. The EY decreased by 0.15 g·kW1·h1, which was lower than in the

DDBD reactor.

As shown in Figure 7, the maximum concentration of ozone increased gradually as the

residence time increased. As the maximum temperature in the reaction system could reach above

100 °C, the ozone participated in toluene degradation and was decomposed by the heat [44]. As the

residence time increased, there were more reactions between pollutant molecules and active

radicals in the reactor. At the same time, more active oxygen atoms reacted with each other to form

ozone, but the increase of ozone generation was slow.

0.25 0.30 0.35 0.40 0.50

0

50

100

150

200

O3

(m

g

m-3

)

Residence time (s)

Figure 7. Effect of the residence time on O3 concentration.

2.4. Gas-Phase Product Analysis

The degradation of VOCs by nonthermal plasma is a complex reaction process that is difficult

to control, so the generation of intermediate by-products is inevitable. In order to investigate the

effect of flow reversing on toluene degradation, the by-products generated in the reverse-flow

DBD-catalyst reactor were investigated.

2.4.1. CO2

In the degradation of toluene, the amount of CO2 produced reflects the degree of toluene

degradation. The CO2 selectivity in the process of toluene degradation by DBD and DDBD reactors

was compared, with the results shown in Figure 8.

Under the same discharge energy density, CO2 selectivity was the highest when toluene was

degraded by flow reversing plasma catalysis. Due to the relatively low degradation rate of toluene

in the DBD reactor (I), the CO2 selectivity was also poor at only 12%. Additionally, the CO2

selectivity in the DDBD reactor (III) was not as good as that in the reverse-flow DBD-catalyst

reactor (V) due to the formation of more organic by-products. These results were consistent with

the degradation of toluene under different reaction conditions.

Figure 7. Effect of the residence time on O3 concentration.

2.4. Gas-Phase Product Analysis

The degradation of VOCs by nonthermal plasma is a complex reaction process that is difficult tocontrol, so the generation of intermediate by-products is inevitable. In order to investigate the effect offlow reversing on toluene degradation, the by-products generated in the reverse-flow DBD-catalystreactor were investigated.

2.4.1. CO2

In the degradation of toluene, the amount of CO2 produced reflects the degree of toluenedegradation. The CO2 selectivity in the process of toluene degradation by DBD and DDBD reactorswas compared, with the results shown in Figure 8.

Catalysts 2020, 10, 511 9 of 17Catalysts 2020, 10, x FOR PEER REVIEW 9 of 17


0

5

10

15

20

(Ⅰ) (Ⅱ) (Ⅲ) (Ⅳ) (Ⅴ)

CO

2 se

lect

ivit

y (

%)

Figure 8. CO2 selectivity in different reactors.

2.4.2. NO2

Since the process of plasma degradation of VOCs is very dependent on the input of energy,

higher energy input can increase the degradation rate of VOCs, but more NO2 will be formed at the

same time. In the process of plasma treatment with air as the carrier gas, it is inevitable that

nitrogen oxides will be generated. The NO2 concentration in the gas was detected using a flue gas

analyzer. The generation conditions for different reactor types were compared, as shown in Figure

9.

0

60

120

180

240

300

NO

2 (

mg

m

-3)

(Ⅰ) (Ⅱ) (Ⅲ) (Ⅳ) (Ⅴ)

Figure 9. Concentration of NO2 formed in different reactors.

It can be seen from Figure 9 that less NO2 was generated in the DBD reactor (I) than in the

DDBD reactor (III). In the DBD reactor, the amount of NO2 production decreased significantly from

255.2 mg·m3 to 134.7 mg·m3 after the introduction of the catalyst. When also combined with flow

reversing, the amount of NO2 production further decreased by 3.28 mg·m3. In the DDBD reactor,

the amount of NO2 production decreased by 68.5 mg·m3 after the addition of the catalyst, so it

could be inferred that the formation and degradation of NO2 were mainly related to the presence of

the catalyst. There was no obvious effect of heat accumulation by the introduction of flow reversing

technology.

2.4.3. Organic By-Products

In the study, infrared analysis and gas chromatographymass spectroscopy (GC-MS) were

used to detect the types of organic by-products generated in the exhaust gas under different

reaction conditions. According to the different vibration frequencies in the infrared spectrum, the

gas-phase product categories were determined.

As shown in Figure 10 and Table 3, ketones, acids, alcohols, and esters were the main organic

by-products generated during toluene degradation. DBD reactor (I) and DDBD reactor (III)

produced a greater concentration and more different types of by-products, and there were more


Under the same discharge energy density, CO2 selectivity was the highest when toluene wasdegraded by flow reversing plasma catalysis. Due to the relatively low degradation rate of toluene inthe DBD reactor (I), the CO2 selectivity was also poor at only 12%. Additionally, the CO2 selectivity inthe DDBD reactor (III) was not as good as that in the reverse-flow DBD-catalyst reactor (V) due to theformation of more organic by-products. These results were consistent with the degradation of tolueneunder different reaction conditions.

2.4.2. NO2

Since the process of plasma degradation of VOCs is very dependent on the input of energy, higherenergy input can increase the degradation rate of VOCs, but more NO2 will be formed at the same time.In the process of plasma treatment with air as the carrier gas, it is inevitable that nitrogen oxides willbe generated. The NO2 concentration in the gas was detected using a flue gas analyzer. The generationconditions for different reactor types were compared, as shown in Figure 9.



0

5

10

15

20

(Ⅰ) (Ⅱ) (Ⅲ) (Ⅳ) (Ⅴ)

CO

2 se

lect

ivit

y (

%)


2.4.2. NO2

Since the process of plasma degradation of VOCs is very dependent on the input of energy,

higher energy input can increase the degradation rate of VOCs, but more NO2 will be formed at the

same time. In the process of plasma treatment with air as the carrier gas, it is inevitable that

nitrogen oxides will be generated. The NO2 concentration in the gas was detected using a flue gas

analyzer. The generation conditions for different reactor types were compared, as shown in Figure

9.

0

60

120

180

240

300

NO

2 (

mg

m

-3)

(Ⅰ) (Ⅱ) (Ⅲ) (Ⅳ) (Ⅴ)


It can be seen from Figure 9 that less NO2 was generated in the DBD reactor (I) than in the

DDBD reactor (III). In the DBD reactor, the amount of NO2 production decreased significantly from

255.2 mg·m3 to 134.7 mg·m3 after the introduction of the catalyst. When also combined with flow

reversing, the amount of NO2 production further decreased by 3.28 mg·m3. In the DDBD reactor,

the amount of NO2 production decreased by 68.5 mg·m3 after the addition of the catalyst, so it

could be inferred that the formation and degradation of NO2 were mainly related to the presence of

the catalyst. There was no obvious effect of heat accumulation by the introduction of flow reversing

technology.


In the study, infrared analysis and gas chromatographymass spectroscopy (GC-MS) were

used to detect the types of organic by-products generated in the exhaust gas under different

reaction conditions. According to the different vibration frequencies in the infrared spectrum, the

gas-phase product categories were determined.

As shown in Figure 10 and Table 3, ketones, acids, alcohols, and esters were the main organic

by-products generated during toluene degradation. DBD reactor (I) and DDBD reactor (III)

produced a greater concentration and more different types of by-products, and there were more


It can be seen from Figure 9 that less NO2 was generated in the DBD reactor (I) than in theDDBD reactor (III). In the DBD reactor, the amount of NO2 production decreased significantly from255.2 mg·m−3 to 134.7 mg·m−3 after the introduction of the catalyst. When also combined with flowreversing, the amount of NO2 production further decreased by 3.28 mg·m−3. In the DDBD reactor, theamount of NO2 production decreased by 68.5 mg·m−3 after the addition of the catalyst, so it could beinferred that the formation and degradation of NO2 were mainly related to the presence of the catalyst.There was no obvious effect of heat accumulation by the introduction of flow reversing technology.


In the study, infrared analysis and gas chromatography-mass spectroscopy (GC-MS) were used todetect the types of organic by-products generated in the exhaust gas under different reaction conditions.

Catalysts 2020, 10, 511 10 of 17

According to the different vibration frequencies in the infrared spectrum, the gas-phase productcategories were determined.

As shown in Figure 10 and Table 3, ketones, acids, alcohols, and esters were the main organicby-products generated during toluene degradation. DBD reactor (I) and DDBD reactor (III) produceda greater concentration and more different types of by-products, and there were more polymerby-products. After the addition of the catalyst to the system, the types of by-products decreasedin reactors (II) and (IV). After the addition of catalyst and flow reversing the system, the amountof by-products produced in reactor (V) was significantly decreased and there were less types ofby-products; mainly ketones and alcohols were produced. This indicated that the optimized systemwas conducive to the degradation of toluene and decreased the generation of organic by-products,thereby decreasing secondary pollution.



polymer by-products. After the addition of the catalyst to the system, the types of by-products

decreased in reactors (II) and (IV). After the addition of catalyst and flow reversing the system, the

amount of by-products produced in reactor (V) was significantly decreased and there were less

types of by-products; mainly ketones and alcohols were produced. This indicated that the

optimized system was conducive to the degradation of toluene and decreased the generation of

organic by-products, thereby decreasing secondary pollution.

(a) (b)

(c) (d)

(e)

Figure 10. Gas chromatographymass spectroscopy (GC-MS) results from different reactors. (a) DBD

reactor; (b) DBD-catalyst reactor; (c) DDBD reactor; (d) DDBD-catalyst reactor; (e) Reverse-flow

DBD-catalyst reactor.

Table 3. Main products in different reactors. DBD, dielectric barrier discharge; DDBD, double

dielectric barrier discharge.

No. Reactors Main Products

Ⅰ DBD acetone, butanone, ethyl acetate, valeric acid, acetyl

methacrylate

Figure 10. Gas chromatography-mass spectroscopy (GC-MS) results from different reactors. (a) DBDreactor; (b) DBD-catalyst reactor; (c) DDBD reactor; (d) DDBD-catalyst reactor; (e) Reverse-flowDBD-catalyst reactor.

Catalysts 2020, 10, 511 11 of 17

Table 3. Main products in different reactors. DBD, dielectric barrier discharge; DDBD, double dielectricbarrier discharge.

No. Reactors Main Products

I DBD acetone, butanone, ethyl acetate,valeric acid, acetyl methacrylate

II DBD-catalyst butanone, ethyl acetate,methylstyrene

III DDBD acetone, butanone, valeric acid,ethyl phenylacetate, decan-1-one

IV DDBD-catalyst acetone, butanone, ethyl acetate,phenethyl alcohol

V Reverse-flow DBD-catalyst butanone, heptanol

Compared with the DBD reaction system, there were more by-products in the DDBD reactionsystem, and the transformation of toluene was highly incomplete. The generation of organic by-productswas not significantly improved in the process of collaborative catalysis, and the degradation of pollutantsand CO2 selectivity were not good. It was suspected that the active sites on the catalyst surface werecovered by a large number of organic by-products, leading to deactivation of the catalyst and evenfurther blocking of the catalyst pores, resulting in poor degradation of pollutants and more by-productgeneration. The experimental results showed that the least amount and types of organic by-productswere produced in the flow reversing DBD catalyst reactor, with this system most effectively inhibitingby-product generation of all the reactors tested.

2.5. Reaction Mechanism

The reaction process of nonthermal plasma co-catalysis for VOC degradation is complex, and thereaction mechanism has not been completely elucidated. Nonthermal plasma plays an important rolein the actual reaction process. Applying knowledge of the reaction mechanism to this study, electronscollided with toluene gas and with other particles, inelastically, to transfer most of their kinetic energyto the particles. This changed the state or structure of the particles, thus producing a large number ofexcited active radicals [45]:

e + O2→ 2 O• + e (1)

e + H2O→ OH• + H• + e (2)

e + N2→ 2 N• + e (3)

The toluene molecules were destroyed by the attack of high-energy electrons and gas-phaseradicals (such as O• and OH•) [12,46]. Therefore, in this experiment, aluminum foil as the groundingelectrode produced a uniform and dense electric field, high-energy electrons, and active radicals, whicheffectively promoted the degradation of toluene. Due to the low bond energy of both the C–H bondsof the methyl group attached to the benzene ring, and the C–C bond between the benzene ring andmethyl group (C–H bond: 3.5 eV; C–C bond: 3.8 eV), the attack of high-energy electrons and activeradicals broke these C–H and C–C bonds, thus forming benzene ring derivatives and other organicintermediates. Benzoic acid, phenylacetic acid, phenylacetone, and other substances were obtainedfrom the product analysis results. As the carbon atoms of the benzene ring were bonded in a conjugatedπ-bond, the benzene ring structure was relatively stable [47]. However, under the double attack ofactive radicals and high-energy electrons, the benzene ring was broken. After ring opening, ether, acid,ketone, and other by-products were further generated, such as acetone, ethyl acetate, and so on. At thesame time, some of the intermediate products reacted further to form more complex structures.

In nonthermal co-catalysis, the plasma and the catalyst acted on the gas component by producingplasma chemical reactions and lowering the surface barrier of the catalyst, respectively. Plasma-activated

Catalysts 2020, 10, 511 12 of 17

gas molecules formed free radicals and influenced the catalytic process. The physicochemical propertiesof the catalyst, such as roughness and dielectric constant, affected the electric field distribution near thecatalyst, thus affecting the rate of the gas-phase process [48,49]. Thus, the plasma and the catalyst hada synergistic effect [13].

The combination of plasma and catalyst mainly affected the degradation efficiency, energy yield,and types of pollutants formed. After the catalyst was added to the system, the form of dischargechanged to a combination of surface discharge of the catalyst and weak micro-discharge in the gapspace [50], resulting in decreased toluene conversion efficiency. Only when the generated surfacedischarge could increase the activity of the catalyst could its negative impact be compensated for bycatalytic action [51]. This also explained why there was direct impact on the degradation of pollutantsin the DDBD catalytic reactor when the catalyst surface became coated, resulting in its inactivation.

In this experiment, the introduction of manganese catalyst resulted in O3, O2, high-energyelectrons, and active radicals in the gas adsorbing onto the catalyst surface. In the process of catalyticdegradation of toluene by nonthermal plasma, high-energy electrons reacted with oxygen in theexhaust gas to form oxygen radicals, and then ozone. As the electron affinity of O3 (2.1 eV) is muchhigher than that of O2 (0.44 eV), O3 can be used as an electron acceptor to promote the formation ofhighly oxidized oxygen atoms [52]. Therefore, O3 was not only a by-product, but also a strong oxidant,which played an important role in the destruction of toluene.

Among the various kinds of active radicals produced, oxygen radicals (O•) and hydroxyl radicals(OH•) had the strongest oxidative ability. In the reaction process, these could react directly withtoluene molecules and intermediate products adsorbed on the catalyst surface [53]. The role of ozonein the catalytic oxidation process was mainly as an electron acceptor, producing more hydroxylradicals and decreasing the recombination rate of electron-hole pairs, thus accelerating the formationof hydroxyl radicals.

With the continuous function of radicals, benzene rings were further decarburized to formstraight-chain organic intermediates, and ultimately CO2 and H2O were produced. The reaction rate ofthe catalyst surface depended on the chemical adsorption of toluene, the type of chemisorbed oxygen,Mn–O bond strength, and the conversion rate between different valence manganese oxides [48,50].Manganese-based catalysts can decompose active O3 [54,55]. In this study, the catalyst adsorbed O3 atits active sites, decomposed it into O• and O2, part of which reacted with adsorbed oxygen species andthe other part oxidized toluene or organic by-products, as follows [47,52]:

X*+ O3→ X + O* + O2 (4)

O3 + O*→ 2 O2 + * (5)

Here, X and * stand for catalysts and active sites, respectively.Nonthermal plasma can induce the dielectric heating of the catalyst, thereby improving its

apparent catalytic activity [10]. Additionally, the combination of flow reversing increased the systemtemperature, further increasing catalyst activity. The mechanism of toluene degradation by theDDBD-catalyst reactor was consistent with that of the DBD-catalyst reactor. The key factor in thereaction was the generation of various active substances, such as high-energy electrons, hydroxylradicals, and oxygen radicals [56]. According to the above results and discussion, the total reactionmechanism for toluene degradation by non-thermal plasma co-catalysis technology is shown inFigure 11 [51].

Catalysts 2020, 10, 511 13 of 17Catalysts 2020, 10, x FOR PEER REVIEW 13 of 17


Figure 11. Reaction mechanism for toluene degradation by nonthermal plasma co-catalysts.

3. Experimental

3.1. Reaction Process

The reaction device is shown in Figure 12. The DBD and DDBD reactor outer tubes were quartz

glass tubes 32 mm in diameter and 1.5 mm in wall thickness. The DDBD reactor inner tubes were 8

mm in diameter and had 1 mm wall thickness. An aluminum foil-wound reaction tube was used as

the grounding electrode, with an effective discharge length of 50 mm. A tungsten wire was used as

the high voltage electrode through the center of the reaction tube, with a diameter of 1.5 mm. The

outside of the reactor was wrapped with insulating cotton. In the experiment, an alternating current

(AC) power supply with variable frequency and variable voltage was used; frequency regulating

range was 503000 Hz and the voltage regulating range was 0100 kV. The discharge duration was 2

h. Three test positions were chosen to monitor the temperature at different positions in the reactor:

O, in the middle in the discharge area; P and Q, in the middle of the heat storage section.

13P O Q

1

3

2

45

76

8 9

10

11

12

14

15

16

M M

M M

a b

c d

P O Q

11

1817

45

76

8 9

10

1215

16

(a) (b)

Figure 12. (a) DBD reactor and (b) DDBD reactor. Schematic diagram of reaction device: 1. quartz

tube; 2. gas pipeline; 3. solenoid valve; 4. high voltage; 5. insulating plug; 6. grounding pole; 7.

discharge area (catalyst section); 8. heat storage section; 9. regenerator; 10. insulating cotton; 11. main

air inlet; 12. main air outlet; 13. reactor inlet (outlet); 14. reactor outlet (inlet); 15. thermocouple

temperature measuring position; 16. power supply; 17. inner tube of DDBD reactor; 18. outer tube of

DDBD reactor.

3.2. Flow-Reversal Device

As shown in Figure 12a, the flow-reversal device controlled the opening and closing of the

pipeline through the solenoid valve. When only solenoid valves ‘a’ and ‘d’ were opened, the gas

passed through the reactor from left to right; when only the solenoid valves ‘c’ and ‘b’ were opened,


3. Experimental


The reaction device is shown in Figure 12. The DBD and DDBD reactor outer tubes were quartzglass tubes 32 mm in diameter and 1.5 mm in wall thickness. The DDBD reactor inner tubes were8 mm in diameter and had 1 mm wall thickness. An aluminum foil-wound reaction tube was used asthe grounding electrode, with an effective discharge length of 50 mm. A tungsten wire was used as thehigh voltage electrode through the center of the reaction tube, with a diameter of 1.5 mm. The outsideof the reactor was wrapped with insulating cotton. In the experiment, an alternating current (AC)power supply with variable frequency and variable voltage was used; frequency regulating range was50–3000 Hz and the voltage regulating range was 0–100 kV. The discharge duration was 2 h. Three testpositions were chosen to monitor the temperature at different positions in the reactor: O, in the middlein the discharge area; P and Q, in the middle of the heat storage section.




3. Experimental


The reaction device is shown in Figure 12. The DBD and DDBD reactor outer tubes were quartz

glass tubes 32 mm in diameter and 1.5 mm in wall thickness. The DDBD reactor inner tubes were 8

mm in diameter and had 1 mm wall thickness. An aluminum foil-wound reaction tube was used as

the grounding electrode, with an effective discharge length of 50 mm. A tungsten wire was used as

the high voltage electrode through the center of the reaction tube, with a diameter of 1.5 mm. The

outside of the reactor was wrapped with insulating cotton. In the experiment, an alternating current

(AC) power supply with variable frequency and variable voltage was used; frequency regulating

range was 503000 Hz and the voltage regulating range was 0100 kV. The discharge duration was 2

h. Three test positions were chosen to monitor the temperature at different positions in the reactor:

O, in the middle in the discharge area; P and Q, in the middle of the heat storage section.

13P O Q

1

3

2

45

76

8 9

10

11

12

14

15

16

M M

M M

a b

c d

P O Q

11

1817

45

76

8 9

10

1215

16

(a) (b)

Figure 12. (a) DBD reactor and (b) DDBD reactor. Schematic diagram of reaction device: 1. quartz

tube; 2. gas pipeline; 3. solenoid valve; 4. high voltage; 5. insulating plug; 6. grounding pole; 7.

discharge area (catalyst section); 8. heat storage section; 9. regenerator; 10. insulating cotton; 11. main

air inlet; 12. main air outlet; 13. reactor inlet (outlet); 14. reactor outlet (inlet); 15. thermocouple

temperature measuring position; 16. power supply; 17. inner tube of DDBD reactor; 18. outer tube of

DDBD reactor.


As shown in Figure 12a, the flow-reversal device controlled the opening and closing of the

pipeline through the solenoid valve. When only solenoid valves ‘a’ and ‘d’ were opened, the gas

passed through the reactor from left to right; when only the solenoid valves ‘c’ and ‘b’ were opened,

Figure 12. (a) DBD reactor and (b) DDBD reactor. Schematic diagram of reaction device: 1. quartz tube;2. gas pipeline; 3. solenoid valve; 4. high voltage; 5. insulating plug; 6. grounding pole; 7. dischargearea (catalyst section); 8. heat storage section; 9. regenerator; 10. insulating cotton; 11. main air inlet;12. main air outlet; 13. reactor inlet (outlet); 14. reactor outlet (inlet); 15. thermocouple temperaturemeasuring position; 16. power supply; 17. inner tube of DDBD reactor; 18. outer tube of DDBD reactor.


As shown in Figure 12a, the flow-reversal device controlled the opening and closing of the pipelinethrough the solenoid valve. When only solenoid valves ‘a’ and ‘d’ were opened, the gas passedthrough the reactor from left to right; when only the solenoid valves ‘c’ and ‘b’ were opened, the gaspassed through the reactor from right to left. In this way, the flow direction of the gas was controlled.Five kinds of reactors were involved in the experiment.

Catalysts 2020, 10, 511 14 of 17

3.3. Filling Materials

The filling materials in the reactor included regenerator and catalyst. The regenerator used in theexperiment was cordierite honeycomb ceramic (400 mesh, Φ29 mm, d = 12.5 mm). The catalyst was7.5 wt % Mn/cordierite, which was placed in the discharge area with a bed length of 50 mm.

3.3.1. Catalyst Preparation

The 7.5 wt % Mn/cordierite catalyst was prepared using the impregnation method with cordieritehoneycomb ceramic of the same specification as the support. The specific preparation process was asfollows. After the cut cordierite carrier was cleaned with oxalic acid and deionized water, the surfacewas coated with silica sol and dried. Certain amounts of manganese nitrate solution and γ-Al2O3powder were mixed evenly in deionized water by stirring for 3 h in a water bath at 60 ◦C to obtain theprecursor slurry. After ultrasonic stirring at 60 ◦C for 6 h, the catalyst was prepared by dipping thecarrier in the slurry and baking it at 110 ◦C for 2 h, then calcining in a muffle furnace at 180 ◦C for 1 hand at 500 ◦C for 3 h.

3.3.2. Catalyst Characterization

The specific surface area, pore volume, and pore size of the samples were determined using aspecific surface analyzer (Gemini V, Micromeritics, Norcross, GA, USA) by the Brunauer-Emmett-Teller(BET) method and the Barret-Joyner-Halenda (BJH) method. Scanning electron microscopy (SEM) wasperformed using the S-4300 microscope (Hitachi, Tokyo, Japan). ICP-OES analysis was performedusing an atomic emission spectrometer (IRIS Intrepid ER/S, Thermo, Waltham, MA, USA).

3.4. Testing Methods

A saturated stream of pure toluene solution in a constant temperature water bath was swept byair prior to entering the gas mixing cylinder. The air passed through the drying pipe in advance ofentering. The mixed gas entered the flow direction changing device after adjustment by the massflowmeter. The toluene concentration was maintained at 600 mg·m−3.

Toluene concentration was determined by gas chromatography (GC; 6890 N, Agilent, Palo Alto,CA, USA), and ozone content was monitored by an ozone analyzer (106-M, 2B Technology, Boulder, CO,USA). The formation of gaseous products such as CO2 and NO2 was detected by a flue gas analyzer(Testo 350M, Lenzkirch, Germany), and the organic by-products of the reaction were determinedby infrared spectrometry (Nicolet iS5, Thermo, Waltham, MA, USA) and gas chromatography-massspectroscopy (GC-MS; Trace DSQ, Thermo, Waltham, MA, USA).

The main parameters involved in the experiment were as follows. According to the concentrationof toluene determined by GC, η was used to indicate toluene conversion, %. Specific energy density(SED) was used to express the discharge energy density of injected unit reaction gas, J·L−1. Energy yield(EY) was used to express the amount of pollutants removed per unit energy consumption, g·kW−1·h−1.The calculation formulas are detailed in the literature [34], and are as follows:

η =C1 −C2

C1× 100% (6)

SED =Umax√

2F(7)

EY = 3.6× (C1 −C2)SED

(8)

CO2 selectivity =CCO2

7(C1 −C2)(9)

Catalysts 2020, 10, 511 15 of 17

Here, C1 and C2 represent the concentration value of toluene import and export, respectively,mg·m−3; F is the gas flow rate, L·min−1; Umax is he voltage peak value, kV; and I is the secondarycurrent value, mA.

4. Conclusions

In order to make full use of the heat in nonthermal plasma systems and decrease the generationof by-products, a reverse-flow nonthermal plasma reactor coupled with catalysts was used for theabatement of toluene. With extension of the commutation period and shortening of the residence time,the system temperature gradually decreased, which was not conducive to the degradation of toluene.Therefore, it was necessary to select appropriate flow direction conversion conditions to make fulluse of the heat. Under DDBD reaction conditions, the degradation efficiency of toluene was higherthan under DBD, but the concentration of NO2 and ozone was relatively high. The selectivity of NO2generated in the DBD reactor was lower, and there was decreased NO2 generation in the DBD reactorafter the introduction of flow direction conversion. When catalyst was added, the concentrationsof NO2 and ozone decreased markedly. The organic by-products in the tail gas mainly includedaromatics, acids, and ketones. In the reverse-flow nonthermal plasma reactor coupled with catalysts,the selectivity for CO2 was the highest, and the selectivity and amount of NO2 were the lowest.

Author Contributions: Conceptualization, W.L.; methodology, W.L. and H.S.; validation, W.L. and H.S.; formalanalysis, W.L., H.S. and X.S.; data curation, H.S., X.S. and Y.Z.; writing—original draft preparation, H.S.;writing—review and editing, W.L.; supervision, W.L.; project administration, W.L.; funding acquisition, W.L.All authors have read and agree to the published version of the manuscript.

Funding: This research was funded by The National Key Research and Development Program of China, grantnumber 2018YFC1903105 and Beijing Major Science and Technology Projects, grant number Z191100009119002.

Conflicts of Interest: The authors declare no conflicts of interest.

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